There's about 10 million "second homes" in the USA by some estimates, but that doesn't necessary say how they're actually used - some are vacation homes that are empty when not used (and are in locations that are effectively "worthless" except for vacationing) whereas others will be a technically owned second home but actually inhabited by a child or relative of the owner.

The mortgage interest lack-of-deduction for most (because of the increase in the standard deduction) also affects things. What we likely should be doing is making it easier to build second homes and "dump" them on the market even for a bit of a profit, so as to increase supply.